Ghrelin is a gastric hormone that acts on pituitary and hypothalamus to stimulate growth hormone release regulating adiposity and appetite. Its activity is thought to be dependent on acylation. Ghrelin is O-octanoylated at Ser3 (acylated ghrelin [AG]) by ghrelin O-acyltransferase (GOAT), a posttranslational modification required for mediating its action through growth hormone secretagogue receptor 1a (GHS-R1a). However, as only 5–20% of circulating ghrelin is acylated, unacylated ghrelin (UnAG, also called des-acyl ghrelin) remains the major circulating form (1). UnAG has long been considered as a degradation product of ghrelin. Hence, UnAG has not received much attention regarding its biological significance and possible therapeutic effects. In recent years however, there has been increasing interest in deciphering the biological actions of UnAG in an attempt to use it as a therapeutic tool (2).

A few years ago in a very well-performed study, Delhanty et al. (2) showed rapid effects of UnAG on genome-wide expression patterns in the adipose tissues, muscles, and livers of GHSR1a knockout mice. These studies established the notion that UnAG has GHS-R1a–independent actions (3). Over the past few years, various groups have demonstrated effects of UnAG on ghrelin antagonism, insulin sensitivity, metabolism, muscle regeneration, and β-cell protection (2,4,5).

In this issue of Diabetes, Togliatto et al. (6) demonstrated the novel actions of UnAG. Using glucose-intolerant ob/ob mice, a model of peripheral arterial disease, they showed that UnAG has a protective effect on muscle and endothelial cells subjected to ischemia. Such actions were mediated through normalization of sirtuin 1 (SIRT1) and superoxide dismutase 2 (SOD-2) expressions. They further demonstrated that the process also causes SIRT1-mediated p53 and histone 3 lysine 56 deacetylation and reduced vascular cell adhesion molecule 1 (VCAM-1) production and recruitment of inflammatory cells. Finally, through a series of experiments, they showed that all such effects of UnAG are orchestrated through micro-RNA-126 (miR-126). Interestingly, treatment with UnAG did not change large vessel perfusion and the effects were independent of neovascularization. The protective effects of UnAG were, however, results of the expansion of resident stem cells (i.e., satellite cells and antioxidant effects on endothelial cells). This group earlier showed that UnAG mediated nitric oxide–dependent mobilization of endothelial progenitor cells in the bone marrow (7). They also previously demonstrated that UnAG promotes skeletal muscle regeneration via SOD-2–mediated miR-221/222 expression (8).

Glucose-induced oxidative stress is a major initiating factor of tissue damage in the context of chronic diabetes complications (9). In this study, UnAG-induced activation of endothelial oxidative defense appears to play a major role in the prevention of cellular damage (6). Beneficial effects of UnAG treatment also were shown to be mediated through miR-126 upregulation. Such upregulation, by reducing reactive oxygen species and augmenting SIRT1 activity, ultimately prevented cellular senescence. These results are also in keeping with previous studies by other investigators who have shown that UnAG protects microvascular endothelial cells from apoptosis through SIRT1 (10). Furthermore, in the endothelial cells and in other organs affected by chronic diabetes complications, accelerated senescence and SIRT1 downregulation, at least in part mediated by other miRs, have been demonstrated (11,12).

miR alterations play important regulatory roles in several, if not all, physiological and pathological processes (13). Here, Togliatto et al. (6) showed that alteration of miR-126 plays a key role in the regulation SIRT1 as well as SOD-2 and VCAM-1. However, as one miR may target multiple transcripts and one transcript may be regulated by multiple miRs, identification of one miR for the treatment of a systemic disease is a challenge. Nevertheless, such approaches provide opportunities to develop RNA-based therapeutics to target specific disease-causing pathway(s) (Fig. 1).

Figure 1

A diagrammatic representation of the mechanisms of UnAG-mediated restoration of normal endothelial function and myocyte regeneration following ischemia. UnAG treatment of the ischemic muscle caused upregulation of miR-126. This miR changed ischemia-induced altered expressions of SIRT1, SOD-2, and VCAM-1. Concerted effects of these molecules can lead to prevention of ischemic changes and restoration of normal endothelial function. Possibly through cell–cell communication, normalization of endothelial cells causes expansion of the resident stem cells (i.e., satellite cells) and helps myocyte regeneration. Exact mechanisms (marked by “?”) of UnAG-mediated miR-126 upregulation, endothelial–myocyte communication, and possible participation by additional related molecules remain to be explored.

Figure 1

A diagrammatic representation of the mechanisms of UnAG-mediated restoration of normal endothelial function and myocyte regeneration following ischemia. UnAG treatment of the ischemic muscle caused upregulation of miR-126. This miR changed ischemia-induced altered expressions of SIRT1, SOD-2, and VCAM-1. Concerted effects of these molecules can lead to prevention of ischemic changes and restoration of normal endothelial function. Possibly through cell–cell communication, normalization of endothelial cells causes expansion of the resident stem cells (i.e., satellite cells) and helps myocyte regeneration. Exact mechanisms (marked by “?”) of UnAG-mediated miR-126 upregulation, endothelial–myocyte communication, and possible participation by additional related molecules remain to be explored.

Close modal

Endothelial cell damage is one of the key initiating mechanisms for chronic diabetes complications (9,14). The study by Togliatto et al. (6) also demonstrates how endothelial abnormality may affect the regenerative capacity of myocytes, possibly through a cell–cell communication. As always, questions remain as to whether ghrelin will produce similar effects in nondiabetic ischemic diseases or in the overt diabetic mice. From a mechanistic standpoint, it is important to understand the pathways leading to UnAG-induced miR alteration. In addition, whether other SIRT1-targeting miRs or other SIRTs play any role remains to be examined. Nevertheless, the results of this study are important. Identification and characterization of novel biological properties of UnAG has far-reaching implications. Most important, it paves the pathway to develop a potential therapeutic tool to prevent ischemic changes in the muscle in patients with and without diabetes who have peripheral arterial disease.

See accompanying article, p. 1370.

Duality of Interest. No potential conflicts of interest relevant to this article were reported.

1.
Kojima
M
,
Kangawa
K
.
Ghrelin: structure and function
.
Physiol Rev
2005
;
85
:
495
522
[PubMed]
2.
Delhanty
PJ
,
Sun
Y
,
Visser
JA
, et al
.
Unacylated ghrelin rapidly modulates lipogenic and insulin signaling pathway gene expression in metabolically active tissues of GHSR deleted mice
.
PLoS One
2010
;
5
:
e11749
[PubMed]
3.
Granata
R
,
Settanni
F
,
Julien
M
, et al
.
Des-acyl ghrelin fragments and analogues promote survival of pancreatic β-cells and human pancreatic islets and prevent diabetes in streptozotocin-treated rats
.
J Med Chem
2012
;
55
:
2585
2596
[PubMed]
4.
Delhanty
PJ
,
Neggers
SJ
,
van der Lely
AJ
.
Should we consider des-acyl ghrelin as a separate hormone and if so, what does it do?
Front Horm Res
2014
;
42
:
163
174
[PubMed]
5.
Callaghan
B
,
Furness
JB
.
Novel and conventional receptors for ghrelin, desacyl-ghrelin, and pharmacologically related compounds
.
Pharmacol Rev
2014
;
66
:
984
1001
[PubMed]
6.
Togliatto G, Trombetta A, Dentelli P, et al. Unacylated ghrelin induces oxidative stress resistance in a glucose intolerance and peripheral artery disease mouse model by restoring endothelial cell miR-126 expression. Diabetes 2015;64:1370–1382
7.
Togliatto
G
,
Trombetta
A
,
Dentelli
P
, et al
.
Unacylated ghrelin rescues endothelial progenitor cell function in individuals with type 2 diabetes
.
Diabetes
2010
;
59
:
1016
1025
[PubMed]
8.
Togliatto
G
,
Trombetta
A
,
Dentelli
P
, et al
.
Unacylated ghrelin promotes skeletal muscle regeneration following hindlimb ischemia via SOD-2-mediated miR-221/222 expression
.
J Am Heart Assoc
2013
;
2
:
e000376
[PubMed]
9.
Brownlee
M
.
The pathobiology of diabetic complications: a unifying mechanism
.
Diabetes
2005
;
54
:
1615
1625
[PubMed]
10.
Shimada
T
,
Furuta
H
,
Doi
A
, et al
.
Des-acyl ghrelin protects microvascular endothelial cells from oxidative stress-induced apoptosis through sirtuin 1 signaling pathway
.
Metabolism
2014
;
63
:
469
474
[PubMed]
11.
Mortuza
R
,
Chen
S
,
Feng
B
,
Sen
S
,
Chakrabarti
S
.
High glucose induced alteration of SIRTs in endothelial cells causes rapid aging in a p300 and FOXO regulated pathway
.
PLoS One
2013
;
8
:
e54514
[PubMed]
12.
Mortuza
R
,
Feng
B
,
Chakrabarti
S
.
miR-195 regulates SIRT1-mediated changes in diabetic retinopathy
.
Diabetologia
2014
;
57
:
1037
1046
[PubMed]
13.
Sun
K
,
Lai
EC
.
Adult-specific functions of animal microRNAs
.
Nat Rev Genet
2013
;
14
:
535
548
[PubMed]
14.
Khan ZA, Chakrabarti S. Chronic diabetic complications: endothelial cells at the frontline. In Frontiers in Cardiovascular Drug Discovery. Rahman AU, Choudhary I, Eds. Sharjah, United Arab Emirates, Bentham Science Publishers, 2010, p. 121–137